雅鲁藏布江缝合带西段达机翁地幔橄榄岩及铬铁矿的铂族元素特征

Platinum-group element characteristics of the peridotite and podiform chromitite from Dajiweng ophiolite of the western segment of Yarlung-Zangbo suture zone, Tibet

达机翁蛇绿岩位于雅鲁藏布江缝合带的西段北亚带,该蛇绿岩主要由地幔橄榄岩、玄武岩以及硅质岩组成,其中地幔橄榄岩以方辉橄榄岩为主,同时含有少量的纯橄榄岩,纯橄岩主要呈不规则透镜状或团块状分布于方辉橄榄岩中。在达机翁地幔橄榄岩中产出有3种不同类型的铬铁矿,分别为块状,豆状以及浸染状铬铁矿。文章主要对达机翁地幔橄榄岩的方辉橄榄岩及豆荚状铬铁矿进行了研究,结合岩石的主量元素和铂族元素,对地幔橄榄岩和豆荚状铬铁矿的成因以及雅鲁藏布江缝合带的找矿远景进行了探讨。达机翁地幔橄榄岩具有较高的Mg O含量以及较低的Al2O3和Ca O等含量,这种亏损的全岩成分指示了达机翁地幔橄榄岩经历了较高的部分熔融作用,同时方辉橄榄岩的PGEs的总量为23.68×10^-9～31.02×10^-9,高于原始地幔的值,Pd和Cu 2个元素的含量较为分散明显偏离部分熔融曲线,指示了达机翁方辉橄榄岩可能遭受了熔体的改造,在熔体-岩石反应的过程中,导致了富含PPGE的硫化物的加入。达机翁豆荚状铬铁矿为高Cr型铬铁矿,具有IPGE和Rh明显富集以及Pt,Pd明显亏损的特征,不同类型的铬铁矿之间具有一致的PGEs的分配模式。雅鲁藏布江缝合带内大量分布的超镁铁岩体在岩石组合、地球化学特征、成因以及形成时代等方面,均具有相似性,是中国铬铁矿找矿的有利远景区。

Located in the northern subbelt of the western Yarlung Zangbo ophiolite zone, Dajiweng ophiolite is mainly composed of peridotite, basalt and silicious rocks. The peridotite is dominated by harzburgite with minor dunite. Dunite occurs as patches in the peridotites. Podiform chromitite can be found in the harzburgite with dunitic envelope. According to the texture and mineralogy of the chromitite, Dajiweng podiform chromitite can be divided into three types, namely massive chromitite, disseminated chromitite and nodular chromitite. This paper mainly reports the research on the harzburgite and podiform chromitite and makes a discussion on the petrogenesis of the peridotite, the origin of the chromitite and chromitite prospect in the Yarlung Zangbo suture zone combined with the major and platinum group elements. Dajiweng harzburgite has a depleted whole rock composition with higher Mg O content and lower Al2O3, Ca O content than the primitive mantle, indicating that Dajiweng peridotite has experienced a high degree of partial melting. Dajiweng harzburgite has higher PGEs content（23.68×10^-9-31.02×10^-9） than primitive mantle. The Cu and Pd content of Dajiweng harzburgite strongly deviates from the modeled partial melting curves, which is a symbol of melt-rock reaction. During the process of melt- rock reaction, sulfides were enriched with PPGEs and these materials were added to the Dajiweng harzburgite. In the primitive mantle- normalized PGE patterns of podiform chromitite, Dajiweng chromitite shows obvious enrichment of IPGE and Rh, and depletion of Pt and Pd. Podiform chromitites of different types have consistent primitive mantle- normalized PGE patterns. Ultramafic rock in the Yarlung Zangbo ophiolite belt shows remarkable similarities in such aspects as rock assemblage, rock geochemistry, formation age and petrogenesis, suggesting that the Yarlung-Zangbo ophiolite is a promising prospecting area for chromitite.